The invention relates to a brake booster and more specifically to a rapid actuation brake booster that can be applied in particular to motor vehicles.
In automotive braking systems, there is generally a perceptible dead travel at the start of braking when the brake pedal is actuated and during which the driver depresses the brake pedal without any hydraulic pressure being induced effectively in the vehicle braking circuit.
There are systems in existence that are able to reduce this dead travel. Such is the case, for example, of the system described in French Patent Application FR 2 856 363.
This system comprises, as depicted in FIGS. 1 and 2:
a casing 2, of longitudinal axis X, containing a skirt 6 mounted such that it can slide axially in a sealed manner in the casing and which delimits a low-pressure first chamber 8 known as the front chamber and a variable-pressure second chamber 10 known as the rear chamber,                a piston 12 secured to the skirt 6,        a three-way valve 26 which, under the control of a control rod 28, can be used to isolate the front and rear chambers from one another, to place them in communication with one another, or to place the rear chamber at a higher pressure with respect to the low pressure such as atmospheric pressure,        a sleeve 54 sliding axially inside the piston 12, an annular face 48 of this sleeve acting as a first valve seat, known as the equalizing seat, for the three-way valve 26,        a slide valve plunger 32 that has an annular face 50 acting as a second valve seat, known as the intake valve seat, for the three-way valve 26,        a key 84 running at right angles to the axis X through two diametrically opposed openings 86 in the piston and two diametrically opposed slots 88 in the sleeve 54. The key 84 is mounted fixedly in the pneumatic piston 12.        
The way in which a booster such as this works is as follows:
In the rest position (FIGS. 1 and 2), the valve shutter 46 is lifted off the first valve seat or equalizing seat 48 (borne by the sleeve 54) and places the front chamber 8 in communication with the rear chamber 10. The valve shutter 46 presses against the intake valve second seat 50 thus isolating the rear chamber from atmospheric pressure.
At the start of a braking phase, when the driver depresses the brake pedal, the control rod 28 is moved axially forward, the valve shutter 46 then comes to press against the first valve seat 48, isolating the rear chamber from the front chamber, and lifts off the second seat 50 and allows air at atmospheric pressure to be supplied to the rear chamber. Because of the pressure difference between the front chamber and the rear chamber, the skirt 6 and the piston 12 are made to move forward. The first valve seat 48 borne by the sleeve 54 is immobile while the clearance C between the key 84 and the front end of the second slots 88 is not closed up.
The spring 58 keeps the sleeve 54 in a determined axial position relative to the casing of the booster as long as the piston 12 has not covered a determined travel C (FIG. 2). The hydraulic piston of the master cylinder is pushed by the pneumatic piston, which itself carries along the auxiliary piston 126 which moves away from the sensor feeler. Once the pressure in the master cylinder is high enough for the auxiliary piston 126 to be able to overcome the jump spring 128, the auxiliary piston is pushed back toward the sensor feeler until it comes into contact therewith and then passes the reaction on from the hydraulic circuit to the brake pedal.
When the pneumatic piston 12 has covered the travel C (see FIG. 2), the front face 100 of the key 84 which is fixed relative to the pneumatic piston comes to bear against the front end of the slots 88 of the sleeve. The sleeve is then axially connected with the movement of the piston. The valve shutter 46 comes into contact with the intake valve seat 50 and interrupts the supply of air at atmospheric pressure to the rear chamber. The driver then has to depress the brake pedal further in order to increase the intensity of braking.
The travel C is preferably chosen such that it corresponds to the dead travel of the master cylinder, that is to say to the travel that the hydraulic piston has to cover within the master cylinder in order to begin to cause the pressure of the brake fluid to rise in the brakes. As a result, the driver feels through the pedal only the travel needed to close the equalizing valve and open the intake valve and does not perceive the dead travel of the master cylinder. Driver comfort is thus improved, because the driver has the impression of immediate braking.
Thereafter, the system enters the actual braking phase.
In a system such as this it is found that there is a peak load that has to be applied to the pedal when the key 84 has covered the travel C and comes into contact with the front end of the slots 88 of the sleeve 54. The object of the invention is to attenuate this transition which is perceivable at the brake pedal.
One subject of the invention is therefore a brake booster comprising:
a casing of longitudinal axis,
a skirt and pneumatic piston assembly mounted such that it can slide in a sealed manner in the casing along the longitudinal axis, said skirt-piston assembly dividing the interior space of the casing into a low-pressure front chamber and a variable-pressure rear chamber,
a three-way valve actuated by a control rod mounted in a longitudinal passage pierced in the pneumatic piston, said control rod being connected via a first longitudinal end to a brake pedal,
a slide valve plunger able to move, in said piston, along said longitudinal axis under the control of a second end of said control rod, said slide valve plunger allowing the force of the control rod to be applied to a hydraulic piston of a master cylinder, said skirt-piston assembly transmitting a pneumatic boost force to the hydraulic piston of the master cylinder,
a sleeve mounted such that it can slide in a sealed manner in the pneumatic piston along said longitudinal axis over a determined travel.
The three-way valve comprises a first and a second valve seat, and a valve shutter intended to be pressed against the first and/or the second valve seat. The first valve seat is borne by a first longitudinal end of the sleeve. The second valve seat is borne by a first longitudinal end of the slide valve plunger.
According to the invention, the booster also comprises:
an elastic device bearing, on the one hand, against a front face of the piston and, on the other hand, against a shoulder of the sleeve. This elastic device tends to exert a forward force on the sleeve relative to the piston,
a device for transmitting the pressure of the master cylinder to the sleeve which tends to apply a rearward force to the sleeve.
Provision will advantageously be made for the elastic device to comprise a first spring.
According to one preferred embodiment of the invention, the device for transmitting the pressure of the master cylinder comprises an auxiliary piston coaxial with the primary piston of the master cylinder. This auxiliary piston is intended to apply a force to the sleeve via the device that transmits the pressure of the master cylinder. Advantageously, this transmission device comprises a second spring.
According to this embodiment, provision may be made for the second spring to be contained between a first washer that bears against a shoulder of the auxiliary piston and a second washer that bears against a front shoulder of the sleeve.
Furthermore, according to the invention, means are provided for limiting the movement of the sleeve with respect to the pneumatic piston. In this context, the pneumatic piston may then comprise an end-stop shoulder and the sleeve may comprise a travel-limiting component intended to butt against said end-stop shoulder in order to limit the travel of the sleeve inside the piston of the booster.
Provision may also be made for the position of the travel-limiting component on the sleeve to be axially adjustable.
Furthermore, there may also be a bearing piece which bears against the casing of the booster and which has a first part that penetrates an opening in the pneumatic piston and against which this piston is intended to bear, and a second part which enters an opening in the sleeve and which is intended to limit the forward movement of the sleeve when the booster is at rest.
Advantageously, the load of the first spring is lower than the load of the second spring.
One exemplary embodiment of the device of the invention will therefore be described with reference to FIGS. 3 and 4.